1. Technical Field
The present invention relates to a master cylinder used in a brake system of a motor vehicle or the like.
In a master cylinder, a piston is slidably fitted in a cylinder body to which operating fluid is introduced from a reservoir, and the cylinder body defines therein a pressure chamber in which the operating fluid is pressurized according to an operation of the piston. The pressure chamber is connected via a pipe to a hydraulic device of a brake system or the like, such that the hydraulic device is operated according to an operation of the piston. The cylinder body is provided with a supply passage for supplying the operating fluid from the reservoir to the pressure chamber, so as to prevent a pressure in the pressure chamber from becoming negative during an operation of returning the piston or the like.
2. Description of the Related Art
In recent years, development has been in progress for traction control systems for automatically applying brake forces to wheels according to vehicle wheel slip conditions. In such a system, a control pump, which is a part of a hydraulic device, sucks operating fluid from a pressure chamber in a master cylinder and supplies the sucked operating fluid to a wheel brake unit during a traction control. During this operation, in the master cylinder, an amount of operating fluid corresponding to the amount of operating fluid sucked from the pressure chamber is supplied from the reservoir via the supply passage to the pressure chamber. However, the supply of operating fluid is likely to be insufficient when the control pump sucks a large amount of operating fluid.
Conventionally, known master cylinders directed to this problem have a structure adapted to open a separate passage connecting the reservoir and the pressure chamber when the pressure chamber does not contain enough operating fluid (refer, for example, to Japanese Patent Application Public Disclosure No. HEI 11-268629).
Such a master cylinder comprises in a cylinder body thereof a bypass passage for bypassing a supply passage and connecting a reservoir and a pressure chamber. The bypass passage is provided therein with a check valve that is adapted to open when the pressure in the pressure chamber becomes lower than that in the reservoir. The check valve comprises a valve seat member having a valve hole, a valve body adapted to be seated on and move away from a valve seat of the valve seat member, and an urging spring for urging the valve body toward the valve seat. These members are disposed in a valve chamber formed in the bypass passage.
In this conventional master cylinder, however, the valve seat member is fixedly disposed in a valve containing hole, and the valve seat on which the valve body is seated, and a guide portion for slidably guiding the valve body are formed as separate bodies, whereby it is difficult to improve precision with which valve body is seated on the valve seat.
Therefore, an object of the present invention is to make it possible to easily improve precision with which the valve body is seated on the valve seat of the check valve, and provide a master cylinder which can be easily manufactured and in which high precision is realized.